In various manufacturing processes objects are moved from one production phase to another, often along a moving production line conveyor. In many processes, such as electronic circuit fabrication, the object-under-manufacture is carried by a “tray” or other carrier, into which the object, or “substrate”, is seated. This tray serves to align the object and provide a base that allows manufacturing processes to be administered to the object. Often the tray includes a well with clamping structures that maintain the substrate securely on-level within the well. By way of example, an object in a tray can move along a production line from a high-pressure wash station to a finishing station.
In the example of processing substrates in carrying trays, it is possible for one production process to cause the substrate to become displaced in the tray (e.g. the pressure of the wash cycle forces one or more or the substrate corners out of the tray's clamping arrangement). This condition can be defined as a “substrate-out-of-pocket”, or “SOOP”. If a substrate becomes displaced, any subsequent processing steps will be inapplicable, as proper alignment of the substrate within the tray is generally required for the substrate to be properly processed. Thus, identifying substrates that are “out of pocket” is highly desirable.
An out-of-pocket substrate is often challenging to detect through conventional techniques. In many instances, the substrate must remain free of contact, and thus metrologies that directly measure to substrate surface may be unavailable. Techniques that employ visible or near-visible (e.g. IR) light are available where contact is to be avoided. One possible technique is the use of edge and/or feature finding with a machine vision system. However, when the substrate is potentially still aligned with the edges of the well, but no longer level within the well, machine vision edge-finding techniques may fail to determine the displacement of the substrate as they operate on a two dimensional image of a scene.
A variety of other light-based processes can be employed to accurately determine if a substrate (or other object) is displaced with respect to its surroundings. For example a plurality of laser displacement sensors can be arrayed over the inspection area. However this is an expensive solution, requiring a large array of sensors to accurately determine the substrate's orientation. This is because each sensor provides only a single-point absolute measurement. In addition, such a sensor arrangement would be subject to positioning errors and the mechanical tolerances of the surrounding tray.
Another possible technique is to employ laser profiling, where the substrate is moved through a laser line while a measurement of the profile is made. However, this technique is generally too slow when attempting to provide the required accuracy for determining displacement of the substrate. Yet another possible technique is to arrange a light curtain in the path of the substrate. However, light curtains are subject to tolerance errors in tray and/or variability in the conveyance system (e.g. variation in the height of the conveyor as it moves through the fixed-height curtain. Likewise, this approach only provides absolute measurements.
All of the non-contacting, light-based detection techniques described above are more particularly subject to error due to the high level of specular reflection from the substrate and/or surrounding tray (or other object), which causes certain regions of scene to provide unexpected results that can skew the analysis of the object.